Method of charging in electrophotography

ABSTRACT

An electrophotographic material comprising a photoconductive isolating layer on an electrically conductive layer is charged without the need for grounding the sheet by simultaneously applying to the photoconductive side of the sheet a corona discharge of one polarity before the developing station and a corona discharge of the opposite polarity after the developing station.

United States Patent 1191 Sato 5] Dec. 18, 1973 [5 METHOD OF CHARGING IN 3,675,011 7/1972 Silverberg 250 495 ZC CT G A H 3,444,369 5/1969 Malinaric.... 250/495 ZC $519,420 7/1970 Gofie 96/1 R [75] Inv nt r: Masamichi Saitama, Japan 3,412,242 11/1968 Giaimo 250/496 2c Assigneez j Photo Film Ltd. 2965,48] 12/1960 Gundlach 96/1 R Kanagawa, Japan Przmary ExammerRoland E. Martin, Jr. Flledi l 1971 A1t0rney-Gerald .1. Ferguson, Jr. 21 App1.No.: 179,349

[57] ABSTRACT 52 us. (:1. 96/1 c, 96/1 R, 317/262 A, An electrophotographic material comprising a P 317/262 AE 250/495 ZC conductive isolating layer on an electrically conduc- 51 1111.01. 603g 13/02, G03g 13/22 rive layer is charged without the need for grounding 58 Field of Search 96/1 R, 1 c; the sheet y simultaneously pp y to the photocon- 317/262 A, 262 AB; 250/495 ZC ductive side of the sheet a corona discharge of one polarity before the developing station and a corona dis- [56] References Cit d charge of the opposite polarity after the developing UNITED STATES PATENTS Siam 3.041,!67 6/1962 Blakney et a1. 96/1.4 3 Claims, 7 Drawing Figures METHOD OF CHARGING IN ELECTROPI-IOTOGRAPHY BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to a novel charging method in the electrophotographic process suitable for charging the electrophotographic material.

2. Description of the Prior Art As a flexible elongated electrophotographic material which comprises a photoconductive insulating layer formed on an electrical conductor, the materials schematically shown in FIGS. 1 through 3 have been known. As shown in FIG. I, the electrophotographic material is composed of a photoconductive insulating layer 2, an electrically conductive layer 3 and a high insulating substrate 4. The photoconductive insulating layer 2 may be made from an amorphous selenium, a mixture of photoconductive particles (such as zinc oxide, cadmium sulfide, etc.) and an insulating resin or an organic photoconductor. The electrically conductive layer 3 may be a metal plating film, a vacuum evaporated metal film, an electrically conductive metal compound film (such as copper iodide, silver iodide, etc.), an electrically conductive high molecular compound film or an electrically conductive paint film (such as a mixture of a resin and a powder of silver or an electrically conductive carbon, etc.). Examples of the high insulating substrate 4 include the films of polyethylene terephthalate, polyethylene, polyvinyl chloride, polypropylene, polycarbonate or triacetyl cellulose, etc.

When the material shown in FIG. 1 is electrically charged by a corona discharge, the electrically conductive layer 3 should be earthed. It is difficult practically, however, to contact an electrode with the side end of a thin electrically conductive layer generally having the thickness of 500 A to several microns. Therefore, in the conventional arts the materials shown in FIGS. 2 and 3 are used.

FIG. 2 is the schematic view in cross section taken along the line of a right angle to the running direction of the electrophotographic material. The photoconductive insulating layer is designated by the reference numeral 2, the electrically conductive layer by numeral 3, the highly insulating substrate by numeral 4, and both of the exposed ends of the electrically conductive layer by numeral 5. In order to expose both ends like this, the end portions of the photoconductive layer may be removed after coating the photoconductive layer over all of the electrically conductive layer as shown in FIG. 1, or the photoconductive layer may be coated on the electrically conductive layer except at the both ends. It is then easy to contact the electrode with the exposed area of the electrically conductive layer 5.

FIG. 3 is the schematic view in cross section illustrating another electrophotographic material used to make better contact with the electrode. Numerals 2, 3 and 4 in FIG. 3 indicate the same members designated by the reference numerals 2, 3 and 4 in FIG. 1. Electrically conductive poles 6 are pierced through the insulating substrate 4, and the upper sides of the poles are connected with the electrically conductive layer 3. The other end of the poles exposed at the back surface of the insulating substrate 4 are contacted with the electrode. In order to make these poles 6, the insulating substrate 4 is pierced by holes into which electrically conductive material is flowed. For instance, the holes may be plated with metal, or an electrically conductive high molecular compound may be flowed into holes.

However, one of the drawbacks of these materials is that the manufacturing processes for the electrophotographic materials shown in FIGS. 2 and 3 are complicated and, therefore, the materials are expensive. When the exposed electrically conductive layer is scratched, the imperfection of contact with the electrode is incidental. The material shown in FIG. 2 has a further drawback in that the both ends of the material where the image cannot be formed, are required to make contact with the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 3 are schematic views of electrophotographic materials while FIG. 4 is an explanatory view of the charging method according to the invention. FIGS. 5 through 7 are schematic views of embodiments of the charging method according to this invention.

SUMMARY OF THE INVENTION This invention offers a charging method which is able to charge well even such an electrophotographic material as shown in FIG. 1.

The charging method according to the present invention is illustrated in FIG. 4. In FIG. 4 the electrophotographic material 1 is being run at a constant speed in the direction of arrow 18. The photoconductive insulating layer 2 is made ofa mixture of zinc oxide powder and an insulating resin. The electrophotographic material is required to be placed under both corona charging devices 10 and 13 and furthermore to be continuous between them. Thus the charging cannot be started until the lead edge of electrophotographic material lands on the position under the corona charging device 13 The high voltage is applied negatively to the corona wire 19 of the charging device 10 and positively to the corona wire 20 of the charging device 13 so that the values of both of the corona currents might be nearly equal. Negative corona ion 21 is deposited on the surface of the photoconductive insulating layer 2 under the corona wire 19 so that a positive charge is induced on the upper surface of the electrically conductive layer 3. On the other hand, the positive corona ion 22 is deposited on the surface of the photoconductive insulating layer under the corona wire 20. Here the photoconductive insulating layer used is a N-type semiconductor so that the free electrons 23 in the electrically conductive layer pass through the photoconductive insulating layer and neutralize the positive charge 22 on the photoconductive surface. It is easy to neutralize the positive charge 22 with many free electrons in the electrically conductive layer. Consequently the positive charge exceedingly increases because of the lack of electrons in the electroconductive layer. The positive charge 24 forms a charge pair with the negative charge 21.

It is important in this invention that the developing apparatus 25 is placed between the charging device 10 and 13. In some embodiments the exposing device may be therebetween. When an image can be exposed before charging, an exposing device is not required to be placed between both of these charging means. A toner image has already been deposited under the charging device 13 on the surface of the electrophotographic material which has pssed through the developing device, so that it is no longer of significance whether it is exposed to light or to a corona discharge. If both charging devices and 13 are placed on the same side against the developing apparatus, that is, the developing device is not set between them, the electrophotographic material is first exposed to a positive corona ion and then to a negative corona ion. For some electrophotographic materials this setting is not suitable because it is difficult for them to be charged with the negative corona ion after exposed to the positive corona ion. If after being exposed to the negative corona ion the electrophotographic material is then exposed to the positive corona ion before developing, the negative charged ion is, of course, neutralized with the positive ion. So this process should not be done. Therefore, the developing device is required to be placed between both charging devices 10 and 13. Description of the Preferred Embodiments Examples according to the invention are described below.

EXAMPLE I This is an example in which the positive development is carried out by an embodiment as shown in FIG. 5. The electrophotographic material 1 is prepared as follows. The surface of a polyethylene terephthalate elongated film having the thickness of 100a and the width of 250 mm, is activated by the projection of ultraviolet rays and the activated surface is then continuously coated with aluminum by vacuum evaporation. Then a mixture of 100 parts by weight photoconductive zinc oxide particles and 20 parts by weight styrenated alkyd resin (trade name, Styresol No. 4400, produced by Japan Leighhold) is coated on the aluminum film at a thickness of 7 after being dried. This prepared electrophotographic material 1 is then charged by the cooperation of charging devices 10 and 13. Viewing a part on the electrophotographic material 1, one area is in the first place exposed to the corona discharge by the first corona charging device 10, during which time the preceding area of it is exposed to another corona discharge by second charging device 13 of counter polarity to the first charging. The area facing the first corona charging device is charged by the principle shown in FIG. 4.

Then the charged electrophotographic material is exposed to the light image in the position 12 and is transported with a pair of transporting rollers 35 and 36 so that the formed latent image is developed. The developer housing is designated by the referenced numeral 15. The dry type developer 37 contained therein comprises two components of a carrier bead and toner clinging triboelectrically to the carrier. The toner is prepared by mixing carbon black and polystyrene in the ratio of l to 8 parts by weight and then powdering the mixture. The carrier bead comprises a coating of nitrocellulose of about 0.1 micron thickness on the surface of a glass bead having the particle size of 0.6 to 0.8 mm. The electrophotographic material that passes the developing unit is then transported to the position of the second corona charging device 13 with a pair of transporting rollers 38 and 39. The first charging device comprises a corona discharge electrode of stainless steel and an earthed shield case 15 mm. apart from the electrode. The charging device is arranged so that there is a clearance between the discharge electrode and the surface of the electrophotographic material. To the corona wire of the first charging device is applied the voltage of -8 KV. and to the corona wire of the second charging device is applied a voltage of +8.5 KV. The running speed of electrophotographic material is 50 mm.sec. The charged electrophotographic material is image-exposed through a transparent line original by a xenon flash lamp of 125 W. sec. output and duration time p.sec. A good copy is obtained.

EXAMPLE 2 This is an example in which an embodiment as shown in FIG. 6 is used. The electrophotographic material is operated around a drum 40 with the speed of 50 mm. sec. As is similar to Example 1 the electrophotographic material is charged, a latent image is formed thereon, and then the latent image is developed by a magnetic drum 41. Magnetic segments 42 are prepared on the surface of a drum composed of non-magnetic material such as aluminum and a magentic brush 43 is formed by the segments. The magnetic drum 41 is rotated in the opposite direction to the drum 40. The brush 43 contacts the surface of the electrophotographic material bearing the latent image for developing it. The developer housing is designated by the reference numeral 15 and the magnetic developer, comprising a magnetic carrier bead and toner, by numeral 44. The toner is the same toner as used in Example 1. The carrier bead comprises coating the nitrocellulose in similar manner as in Example 1 onto the surface of an iron particle having the size of 0.5 to 0.2 mm. The charging voltages are the same as in Example 1. A good copy is also obtained.

EXAMPLE 3 This is an example in which an embodiment as shown in FIG. 7 is used. The electrophotographic material is prepared in the same manner as in Example 1 and is 0perated around a drum 45 at a speed of 50 mm. sec. The charging of the electrophotographic material is carried out by the same method as in Example 1. The container for the liquid developer is designated by the reference numeral 15 and liquid developer by numeral 16. The lower part of the drum 45 dips into the developing liquid. A pair of squeeze rollers 46 and 47 squeeze developing liquid and remove it. A good copy is obtained. The developing liquid is prepared with the following mixture:

Carbon black l0 parts by weight (trade name Nippeal No. produced by Nittetsu Chemical Co., Ltd.)

These are mixed in ball mill for 40 hours and the mixture obtained is dispersed by ultra-sonic energy into a mixing solvent of kerosene and linseed oil to dilute the mixture to 30 times its volume. The ratio of the mixing solvent is 1 liter of kerosene to 100 ml of linseed oil. The output of the ultra-sonic mixer is watts and frequency thereof is 29 KHz. The toner in the developing liquid possesses a negative charge.

In accordance with the above mentioned invention, it is possible to charge the electrophotographic material without earthing the electrically conductive layer. Therefore, even the electrophotographic material shown in FIG. 1 which is difficult to be earthed can be charged without any means as shown in FIGS. 2 and 3 so that a developed image is obtained.

Also it is preferable for effective charging that the electrically conductive layer in the electrophotographic material used for the invention has a volume resistivity of less than IO' Qcm. In the case of less than [0 0cm it is especially effective. In the process according to the invention, because a closed circuit is formed through the electrically conductive layer, an effective operation is not obtained if the resistivity value in longitudinal direction of the electrically conductive layer is over some value. According to the test of using an elongated electrophotographic sheet having the width of about 10 to 100 mm. and having an electrically conductive layer (as shown in above examples) of about 2 to lOp. thickness formed on a paper substrate, good results are obtained even in the case where the distance of two charging devices as shown in FIG. 4 is several meters.

If when starting to make a copy the leading portion of the electrophotographic material does not reach the position of secondary charging device, then the method according to this invention is not effective. In this case an electrically conductive leader may be attached to the lead edge of the electrophotographic material.

This invention is, of course, useful for a conventional electrophotographic material.

What is claimed is:

1. An electrophotographic corona charging method for charging an elongated sheet of an electrophotographic material having an ungrounded electrically conductive base having a volume resistivity of less than 10 ohm-centimeters and a photoconductive insulating layer formed thereon, said electrically conductive base being interposed between said photoconductive insulating layer and an electrically insulating substrate, said method comprising moving said elongated sheet past two corona discharge devices, an image exposing station and a developing station, where a first corona discharge device is in front of said image exposing station and a second corona discharge device is behind said developing station and then simultaneously exposing the photoconductive insulating layer side of said sheet to first and second corona discharges in front of and behind said image exposing and developing stations, said corona discharges having an opposite polarity to each other.

2. The method of claim 1 wherein the corona discharge before developing is positive and the corona discharge after developing is negative.

3. The method of claim 1 wherein the corona discharge before developing is negative and the corona discharge after developing is positive. 

2. The method of claim 1 wherein the corona discharge before developing is positive and the corona discharge after developing is negative.
 3. The method of claim 1 wherein the corona discharge before developing is negative and the corona discharge after developing is positive. 